Lighting systems based on LEDs are used on an increasing scale. LEDs have a high efficiency and a long life time. In many lighting systems, LEDs also offer a higher optical efficiency than other light sources. As a consequence, LEDs offer an interesting alternative for the well-known light sources such as fluorescent lamps, high intensity discharge lamps or incandescent lamps.
The lighting systems based on LEDs often comprise a power supply circuit that supplies power to the LEDs comprised in one or more LED modules that are connected to output terminals of the power supply circuit during operation. Typically the total current supplied by the power supply circuit depends on the number of LED modules connected to power supply circuit and more in particular to the nominal current suitable for each of the LED modules and also on the temperature of the LED modules. The LED modules LM comprised in a LED lighting system called Fortimo manufactured by Philips, that is presently on the market and is schematically shown in FIG. 1, comprise a first resistor Rset having a resistance that represents the nominal current suitable for the LEDs comprised in the LED module, and furthermore comprise a second resistor NTC having a temperature dependent resistance. In case one or more of these LED modules is connected to the power supply circuit PSC, a circuit MC, which is comprised in the power supply circuit PSC, causes a current to flow through the first resistor Rset and another current to flow through the second resistor NTC. The voltages across each of the resistors are measured and the value of the resistance of each of the resistors is determined by the circuit MC from the measured voltage across each of the resistors. From these data, the circuit part MC derives a desired value for the total LED current. A driver circuit DC, which is comprised in the power supply circuit PSC, subsequently adjusts the current supplied to the LED modules to a desired value.
An important disadvantage of this prior art is that three wires are required for connecting the resistors in the LED module to circuitry comprised in the power supply circuit. This makes these existing LED lighting systems rather complex.